1. Field of the Invention
The present invention relates to a thermoplastic molding composition having cold impact strength.
2. Discussion of the Background
The material property specifications which thermoplastic molding compositions must meet in the manufacture of, for example, transport pipelines are quite high. Such structures must be inert to the product being transported and must retain shape stability even under thermal stress. Lines that are used outdoors must withstand weathering. This means that they must essentially not lose their serviceability characteristics when subjected continually to solar radiation, atmospheric humidity, cold, and the like.
In general, all conventional polyamides (PA) are suitable for use in the manufacture of molded parts. Nevertheless, especially when polyamides are used outdoors, there are limitations. Thus, of the industrially used polyamides that have at least one monomer with .ltoreq.10 carbon atoms in the carbon chain, none are known that are not strongly sensitive to water and humidity. The impact strength property, especially the cold impact strength, is a property which substantially deteriorates (Plastics Handbook, 19th Edition, C. Hanser Verlag--pages 294/295, Table).
More suitable are polyamides with a larger number of carbon atoms in the carbon chain, such as for example, PA 11 or PA 12, since they show increased stability to weathering and thus also improved cold impact strength. However, these industrially readily available higher polyamides, when used with plasticizers, suffer significant cold impact strength losses. Normally, an impact strength modifier is added to the polyamide in order to prevent loss of cold impact strength. strength as described, for instance, in DE-AS 2716004. However, one skilled in the art is then forced to work with a three-component system.
Another drawback of PA 11 or PA 12 is that because they are prepared from a lactam or the corresponding aminocarboxylic acid, the crude polymer naturally also contains monomer and oligomer that have to be removed from the product in an additional operating step.
Plasticizers and their use as an additive in polyamides are also known. A need therefore continues to exist for a polyamide molding composition which exhibits improved weathering characteristics.